As used in this article “child” means a member’s child who is dependent upon such member at the time of the member’s death and while such child is unmarried and:
(a) Under l8 years of age, or
(b) Whether under or over 18 years of age, totally disabled and such disability occurred prior to such child attaining age 18 years, or
(c) Eighteen years of age or over, but has not attained the age of 22 years, and is enrolled as a full-time student in an accredited school, as determined by the board.